1. Field of the Invention
The invention relates to a boron oxide-silicon dioxide mixed oxide, a process for its preparation, and its use.
2. Prior Art
Document DE-A 21 22 066 describes the use or pyrogenically produced mixed oxides of boron with silicon, aluminum, titanium and/or iron having a boron content of 2 to 20 wt. %, calculated as elementary boron, as a filler in organopolysiloxane compositions (for example bouncing putty). In particular these mixed oxides contain 5 to 10 wt. % of boron, calculated as elementary boron. These mixed oxides can be prepared by using metals and/or metal compounds according to Ullmanns Enzyclopadie der technischen Chemie, (Ullmanns Encyclopedia of Industrial Chemistry), Vol. 15 (1964), p.726. They are preferably prepared from volatile compounds of boron and silicon, aluminum, titanium and/or iron, especially the chlorides of the aforementioned elements, in the presence of water formed in situ at temperatures above 800.degree. C., i.e. by flame hydrolysis. The mixed oxides obtained have a BET surface of 100 to 400 m.sup.2 /g.
According to the example given in document DE-A 21 22 066, 4.8 kg/h of SiCl.sub.4 and 1.2 kg/h of boron trichloride are vaporised and burnt together with 1.5 m.sup.3 /h of hydrogen in a combustion chamber, under the addition of 4.2 m.sup.3 /h of air. The temperature in the combustion chamber is more than 800.degree. C. The resultant mixed oxide contains 82.5 wt. % of SiO2 and 17.5 wt. % of B.sub.2 O3 (=5 wt. % of boron) calculated as boron and has a BET surface of 180 m.sup.2 /g.
The known boron oxide-silicon dioxide mixed oxide has the disadvantage that it occurs in very finely divided form having a BET surface greater than 100 m.sup.2 /g. It is unsuitable for use as a raw material in glass-making.
The known process has the disadvantage that boron trichloride is used as starting material for the boron oxide. Boron trichloride is a poisonous compound with a boiling point of 12.5.degree. C. When used at room temperature special safety measures are therefore necessary. The commercially available boron trichloride may contain noticeable amounts of poisonous phosgene as impurity.
The object of the invention is therefore to prepare a boron oxide-silicon dioxide mixed oxide that does not have these disadvantages.